Find sigma rule 
Attack: Obfuscated Files or Information: Compile After Delivery
Adversaries may attempt to make payloads difficult to discover and analyze by delivering files to victims as uncompiled code. Text-based source code files may subvert analysis and scrutiny from protections targeting executables/binaries. These payloads will need to be compiled before execution; typically via native utilities such as csc.exe or GCC/MinGW.(Citation: ClearSky MuddyWater Nov 2018)
Source code payloads may also be encrypted, encoded, and/or embedded within other files, such as those delivered as a Phishing. Payloads may also be delivered in formats unrecognizable and inherently benign to the native OS (ex: EXEs on macOS/Linux) before later being (re)compiled into a proper executable binary with a bundled compiler and execution framework.(Citation: TrendMicro WindowsAppMac)
MITRE
Tactic
- defense-evasion
technique
- T1027.004
Test : Dynamic C# Compile
OS
- windows
Description:
When C# is compiled dynamically, a .cmdline file will be created as a part of the process. Certain processes are not typically observed compiling C# code, but can do so without touching disk. This can be used to unpack a payload for execution. The exe file that will be executed is named as T1027.004_DynamicCompile.exe is contained in the ‘bin’ folder of this atomic, and the source code to the file is in the ‘src’ folder. Upon execution, the exe will print ‘T1027.004 Dynamic Compile’.
Executor
powershell
Sigma Rule
-
file_event_win_csharp_compile_artefact.yml (id: e4a74e34-ecde-4aab-b2fb-9112dd01aed0)
-
proc_creation_win_csc_susp_dynamic_compilation.yml (id: dcaa3f04-70c3-427a-80b4-b870d73c94c4)